Orc mutants arrest in metaphase with abnormally condensed chromosomes

The origin recognition complex (ORC) is a six subunit complex required for eukaryotic DNA replication initiation and for silencing of the heterochroma tic mating type loci in Saccharomyces cerevisiae, Our discovery of the Dros ophila ORC complex concentrated in the centric heterochromatin of mitotic c ells in the early embryo and its interactions with heterochromatin protein 1 (HP-1) lead us to speculate that ORC may play some general role in chromo somal folding. To explore the role of ORC in chromosomal condensation, we h ave identified a mutant of subunit 5 of the Drosophila melanogaster origin recognition complex (Orc5) and have characterized the phenotypes of both th e Orc5 and the previously identified Orc2 mutant, k43, Both Orc mutants die d at late larval stages and surprisingly, despite a reduced number of S-pha se cells, an increased fraction of cells were also detected in mitosis, For this Latter population of cells, Ore mutants arrest in a defective metapha se with shorter and thicker chromosomes that fail to align at the metaphase plate within a poorly assembled mitotic spindle. In addition, sister chrom atid cohesion was frequently lost. PCNA and MCM4 mutants had similar phenot ypes to Ore mutants. We propose that DNA replication defects trigger the mi totic arrest, due to the fact that frequent fragmentation was observed. Thu s, cells have a mitotic checkpoint that senses chromosome integrity, These studies also suggest that the density of functional replication origins and completion of S phase are requirements for proper chromosomal condensation .